Geoscience Reference
In-Depth Information
m) crosses the corotation
field at an
L
value near 7. The value corresponding to the higher magneto-
spheric field matches the corotation field at about
L
The lower-value magnetospheric curve (0
.
4mV
/
3. For this range of
magnetospheric sources and the properties of the earth's dipole field and rota-
tion rate, we see that planetary rotation dominates the physics at the equator
within, say, 3-7 earth radii. The “extraterrestrial” source dominates at higher
L
values.
The region referred to as the plasmasphere corresponds to latitudes where the
flux tubes corotate, since such tubes can fill with plasma on the dayside. Sunlight
copiously creates an F-region atomic oxygen plasma, which in turn flows out
along the field lines, exchanging charge with neutral hydrogen atoms. At night
there is not sufficient time to deplete these flux tubes, and, on average, a dense,
cold hydrogen ion plasma fills the upper plasmasphere. The average profile of
plasma density in the magnetospheric equatorial plane reproduced in Fig. 5.7
shows how drastic the decrease in electron density can be near what is called the
plasmapause (at
L
=
4 in this case). Since the magnetospheric electric field is quite
variable, Fig. 5.6 shows that the position of the plasmapause and thus the plas-
masphere's effect on the midlatitude ionosphere as a plasma reservoir are strong
functions of the applied magnetospheric electric field (see Chapter 8). Magnetic
activity and solar wind conditions thus affect the midlatitude ionosphere through
electron dynamics as well as chemistry (because of neutral composition changes)
and, as we shall see, thermospheric winds driven by high latitude heating and
momentum transfer.
=
10
4
10
3
10
2
10
1
10
0
2
3
4
5
6
7
8
Height (in earth radii)
Figure 5.7
Average equatorial profile of electron density. The magnetic condition rep-
resented is one of steady, moderate agitation, with
K
P
in the range 2-4. [After Angerami
and Carpenter (1966). Reproduced with permission of the American Geophysical
Union.]
Search WWH ::
Custom Search